This invention relates to infrared (IR) microscopes, i.e., microscopes which provide for sample, or specimen, illumination by infrared radiation, thus permitting spectral analysis of non-visible radiation wavelengths.
More specifically, the problems which need solution concern IR microscopes in the reflectance mode, in which the sample reflects the radiation, as distinguished from IR microscopes in the transmission mode, in which the radiation passes through the sample.
Providing a microscope for IR reflectance measurements presents some unique challenges. The most significant challenge results from the fact that most of the samples of interest are specular (i.e., highly reflecting). It thus is important to illuminate the sample with radiation that is angled appropriately so that the specular component will reach the IR detector. This need should preferably be accomplished with a design that can be easily switched between transmission and reflection operation.
One way to accomplish specular illumination is to illuminate the sample with off-axis radiation and then to tilt the sample an appropriate amount to achieve the desired result. This approach is generally not desirable from the user's viewpoint, since the sample may often be a small particle of contaminant on a specular surface, and thus might be lost from view if tilted out of the horizontal plane.
A second approach employs a semitransparent beam-splitter which is placed in the optical path. This will typically consist of a thin metallic coating on a transparent substrate such as potassium bromide (KBr). Ideally, the beamsplitter would be 50% transmitting and 50% reflecting. However, practical beamsplitters are more typically 40% transmitting, 40% reflecting, and 20% absorbing. In this approach, the collimated beam from the interferometer is diverted so as to eventually be reflected by the beamsplitter toward the microscope objective. The primary deficiency of such a system is the loss of optical power. Typically, about 40% of the incident beam will be reflected by the beamsplitter toward the sample. After reflection from the sample, the beam must pass through the beamsplitter in order to reach the IR detector. For a typical, good quality beamsplitter, only about 40% will be transmitted. The other 60% will either be absorbed or reflected back toward the interferometer. Thus, as previously stated, the total usable signal, even for otherwise perfect optics, will only be 0.4.times.0.4=0.16 (i.e., 16%) of the incoming optical beam.